The invention relates to an extrusion installation for elongate products, more particularly plastics tubes, having an extruder, followed by a water-cooled calibration sleeve which can be acted upon by negative pressure and a wall thickness measuring device having an ultrasonic sensor.
Normally for the measurement of the wall thickness of the elongate product produced by the extrusion installation, more particularly the wall thickness of the plastics tube, use is made of ultrasonic wall thickness measuring apparatuses which are based on the measurement of the running time of the ultrasonic signal introduced into the product. The propagation time of the ultrasonic signal depends on the properties of the material and the temperature of the extruded product. To obtain as accurate an indication as possible of the wall thickness along the periphery of the elongate product, the temperature of the extruded product must be constant at the measuring positions of the ultrasonic sensors, which are normally disposed uniformly distributed over the whole periphery.
DE 39 06 363 C1 discloses an extrusion installation of the kind specified in which the wall thickness measuring device is disposed between the extruder and a cooling system. Rotating sensors are provided which rotate around a fixed, water-cooled calibration sleeve acted upon by negative pressure. It is true that with such a device the temperature distribution over the periphery can be uniformized, but even in that case intolerable inhomogeneities of temperature can still occur. Moreover, the prior art device is comparatively complicated both in construction and handling, since extensive refitting work is needed for transition from one extrusion diameter to another.
In contrast, it is an object of the invention so to further develop an extrusion installation of the kind specified that on the one hand the accuracy of the wall thickness measuring apparatus is improved, while on the other hand the expense of machinery for the refitting of the installation is reduced.